The demo required that Spencer, a male rhino in his late 20s,  be shot with a tranquilizer gun so that demonstrators could dye and insert a  tracking device into his horn—a procedure meant to deter poachers from slaying  the animal for its horn. But the sedation process failed.

"The rhino had an unfortunate reaction to the anesthesia,"  Lorinda Hern, a  spokeswoman for the Rhino & Lion Nature Reserve told Reuters.  "Every time you dart a rhino, you take a risk that the rhino might not wake up  and unfortunately today was one of those days."